1. Field of the Invention
The present invention relates to a method for manufacturing a discharge tube. More particularly, the present invention relates to a method for manufacturing a discharge tube, characterized by its sealing step for sealing the translucent insulating tube.
2. Description of the Prior Art
Discharge tubes have a pair of electrodes opposing each other inside a quartz tube, which is filled with a certain gas and discharge material. To seal the discharge tube, a portion of the quartz tube is usually softened by heating it and then sealed shut. Generally, an oxygen-hydrogen burner is used for the heat source. However, because it is very difficult to keep the region heated by the burner small, there is a possibility that other portions of the quartz tube besides the portion to be sealed are heated as well and deformed. Especially, deformations of the part serving as the discharge space of the discharge tube are a problem, because they have a large influence on the characteristics of a discharge lamp using this discharge tube. Another problem is the possibility that the hydrogen in the flame of the oxygen-hydrogen burner is absorbed by the quartz glass, which deteriorates the characteristics of the discharge tube.
To solve these problems, it has been proposed to use laser light as the light source for heating the quartz tube (see e.g. Publication of Unexamined Japanese Patent Application (Tokkai) No. S57-109234 or Tokkai S58-78348). For example, Tokkai S58-78348 discloses heating and sealing the quartz tube by irradiating laser light, by scanning it along the tube axis of the quartz tube with a certain amplitude.
However, if the quartz tube is heated by irradiation with laser light that is scanned with a constant amplitude, temperature variations arise in the heating temperature of the quartz tube. A resulting problem is that there is a possibility that the discharge tube bursts during processing, and because there are large variations in the pressure resistance of the seal, there is a possibility of cracks in the discharge tubes occurring during use.
It is an object of the present invention to provide a method for manufacturing a discharge tube including a step of sealing a translucent insulating tube with laser light, wherein variations in the heating temperature of the translucent insulating tube can be reduced, and a discharge tube of high quality can be produced with high reliability.
To achieve these objects, a method for manufacturing a discharge tube in accordance with the present invention comprises irradiating laser light on a portion of a translucent insulating tube, and sealing the portion of the translucent insulating tube onto which laser light has been irradiated. The irradiation of the laser light is performed while (i) scanning the laser light to oscillate back and forth along a tube axis of the translucent insulating tube, and (ii) changing at least one of an intensity of the laser light and an amplitude of the oscillation of the laser light. With this configuration, variations in the heating temperature of the translucent insulating tube can be reduced, and a discharge tube of high quality can be produced with reliability. In addition, it is easy to achieve optimal heating in accordance with several external conditions in the step of heating the discharge tube, because it is possible to heat the translucent insulating tube locally and selectively.
In this method, it is preferable that the irradiation of the laser light is performed while changing the intensity of the laser light such that the intensity of the laser light when a size of a displacement of the laser light is maximal is smaller than the intensity of the laser light when the size of the displacement of the laser light is minimal. Here, xe2x80x9cdisplacementxe2x80x9d refers to a vector expressing the irradiation position of laser light, whose direction corresponds to a direction along the tube axis of the translucent insulating tube, and whose magnitude (i.e. size) corresponds to the distance from an average irradiation position, wherein xe2x80x9caverage irradiation positionxe2x80x9d corresponds, for example, to a central portion of the irradiation region.
FIG. 7 shows the temporal change of the displacement and the intensity of the laser light in conventional step of sealing the discharge tube with laser light. The solid line in FIG. 7 marks the displacement of the laser light, and the broken line marks the intensity of the laser light. FIG. 8 shows the distribution of the irradiation energy of the laser light when the irradiation of the laser light is performed as shown in FIG. 7. If the laser light is oscillated and irradiated with a constant intensity and amplitude, the irradiation energy at the portions where the size of the displacement of the laser light is maximal (i.e. the end portions of the irradiation region near the points xe2x80x9cAxe2x80x9d and xe2x80x9cCxe2x80x9d in FIG. 8) tends to be high, and the irradiation energy at the portions where the size of the displacement of the laser light is minimal (i.e. the center portion of the irradiation region near point xe2x80x9cBxe2x80x9d in FIG. 8) tends to be low. This non-uniformity of the irradiation energy becomes one of the reasons for variations of the heating temperature.
However, in accordance with the above-mentioned preferable manufacturing method of the present invention, the intensity of the laser light is varied depending on the size of the displacement, so that the above-noted non-uniformities in the irradiation energy can be relaxed and variations in the heating temperature of the translucent insulating tube can be reduced.
In this preferable configuration, it is even more preferable that the intensity of the laser light is changed stepwise or continuously such that the intensity of the laser light is reduced with an increase of the size of the displacement of the laser light, and the intensity of the laser light is increased with a reduction of the size of the displacement of the laser light.
In this configuration, it is preferable that the irradiation of the laser light is performed while changing the intensity of the laser light such that the intensity of the laser light is smaller when a scanning speed of the laser light is minimal than when the scanning speed of the laser light is maximal.
As has been mentioned above, the distribution of the irradiation energy of the laser light in conventional manufacturing methods is such that the irradiation energy is high at the ends of the irradiation region and low at the center thereof. The reason for this is that the scanning speed of the laser light at the ends of the irradiation region is low, whereas it is high in the center, because ordinarily, the scanning of the laser light lets the position of the laser light change with a trigonometric function over time. In the regions where the scanning speed is slow, the laser light is irradiated for a longer time then in regions where the scanning speed is high, so that if the intensity of the laser light is constant as in the conventional manufacturing methods, the regions with low scanning speed absorb more energy than the regions with high scanning speed.
However, with the above-noted preferable configuration of the present invention, the intensity of the laser light is varied depending on the scanning speed, so that the above-noted non-uniformities in the irradiation energy caused by the differences in the scanning speed can be reduced and variations in the heating temperature of the translucent insulating tube can be reduced.
In this preferable configuration, it is even more preferable that the intensity of the laser light is changed stepwise or continuously such that the intensity of the laser light is increased with an increase of the scanning speed of the laser light, and the intensity of the laser light is reduced with a reduction of the scanning speed of the laser light.
Furthermore, it is preferable that the irradiation of the laser light is performed while contacting with a coolant a portion of the translucent insulating tube where the laser light is not irradiated. In this case, it is even more preferable that the intensity of the laser light is changed such that the intensity of the laser light when scanning a portion that is closest to a portion of the translucent insulating tube contacting the coolant is larger than the intensity of the laser light when scanning a portion that is farthest away form a portion of the translucent insulating tube contacting the coolant.
If the irradiation of the laser light is preformed while contacting a portion of the translucent insulating tube with a coolant, energy can be dissipated easier from the region that is close to the portion contacting the coolant than from the region further away, so that the heating temperature tends to be lower. However, in the above-noted preferable configuration, the intensity of the laser light when scanning a portion that is closer to a portion contacting the coolant is larger than the intensity of the laser light when scanning a portion that is farther away, so that the energy irradiated on a portion closer to a portion contacting the coolant can be larger, and variations in the heating temperature of the translucent insulating tube can be reduced.
Furthermore, it is preferable that the irradiation of the laser light is performed while rotating the translucent insulating tube around a tube axis of the translucent insulating tube, because this reduces variations in the heating temperature along the circumference of the translucent insulating tube.
The laser light can be emitted by a laser selected from the group consisting of a carbon gas laser, an excimer laser, a YAG laser and a semiconductor laser. A material for the translucent insulating tube is selected from the group consisting of quartz glass, borosilicate glass, and translucent alumina.